Iphone 4 won't play call audio through blueooth, but will play music

I recently got a pioneer deh-p9400bh head unit for my car. My issue is that the phone plays music fine through the usb cable, and fine through bluetooth, but whenever i try to make a call i cannot hear the person on the other line through my car speakers. They can hear me just fine, but the only way i can hear them is if i put them on speakerphone, which plays through the iphone. Ive tried clickin on the pioneer icon that pops up and it doesnt help. Bluetooth is obviously on, as they hear me loud and clear through my stereo's microphone. I've looked through my entire pioneer booklet and have found nothing even close to what i'm looking for. Unfortunately, I do not have any other bluetooth devices that I can try and pair with the radio. I should also add that I am using IOS 5.1.1, as i use the maps function daily for work and do not want to use apple maps. Any ideas? Thanks in advance